Catoptric light collector



Sept. 30, 1969 1.. N. MERTZ 3,469,902

CATOPTRIC LIGHT COLLECTOR Filed Jan. 27, 1966 PRINCIPAtAXIS /&POLAR AXIS 26 b 1 I] i 40 I bl Y 20 I In VEIQ'N )R. LAWRENCE N, MERTZ ATTORNEY United States Patent 3,469,902 CATOPTRIC LIGHT COLLECTOR Lawrence N. Mertz, Lexington, Mass., assignor to Block Engineering, Inc., Cambridge, Mass., a corporation of Delaware Filed Jan. 27, 1966, Ser. No. 523,412 Int. Cl. G02b 5/10, 5/12 US. Cl. 350-294 5 Claims ABSTRACT OF THE DISCLOSURE A light collecting system having a large concave spherical reflector and an apertured secondary reflector having a concave surface directed toward the concave surface of the large reflector. The secondary reflector is aspheric so as to recollimate light received from the large reflector. The secondary reflector is positioned adjacent the paraxial focus of the large reflector so that the latter can act as a tertiary reflector and focus a beam through the aperture of the secondary reflector.

This invention relates to catoptric systems for collecting light, particularly for photometry and spectrometry, and more particularly to a multiple element catoptric system.

It is difficult to make a spectral examination of a dim light source, such as a star, using a large, focussing reflector or dish and an ordinary spectrometer. Even at low focal ratio, a large telescope dish gives a large image; thus the size of the spectrometer slit needs to be increased with the diameter of the dish. It is impossible to feed a large amount of starlight from a large dish through a narrow spectrometer slit; however, widening the slit requires, for the same resolution, a physically large spectrometer. Thus, with a large dish as a light collector, the conventional spectrometer tends to become energy limited rather than diffraction limited. In other words, the throughput of conventional stellar spectrometers tends to limit the useful size of its ancillary light collecting optics.

If, however, one considers interference or Fourier transform spectrometers (such as are described in Transformation in Optics, L. Mertz, John Wiley & Sons, 1965), because they require no input slit, they can accept light from very large light collectors, e.g. with dish diameters of even one hundred feet.

It is, therefore, a principal object of the present invention to provide a light collecting catoptric system for collecting light particularly for use with interference spectrorneters.

Another object of the present invention is to provide such a system comprising a focussing concave primary reflector of large diameter, and an apertured, aspheric concave secondary reflector disposed adjacent the paraxial focus of the primary reflector.

Other objects of the present invention are to provide such a system in which the primary reflector is stationary and thus not subjected to variable gravitational flexure; to provide such a system in which the primary reflector has a spherical curvature and so is relatively easy to figure and test; to provide such a system in which the secondary reflector is mounted coaxially along a radius of the primary reflector between the latter and the center of primary curvature, for rotation about the center; and to provide such a system in which the ratio of aperture to overall size is much larger then for conventional telescopes.

These and other objects are attained by providing a catoptric system comprising a first spherically concave reflector fixedly positioned with respect to the earths surface for receiving and focussing radiant energy from a ice distant source; a second, aspherically concave, centrally apertured reflector positioned adjacent the paraxial focus of the first reflector for receiving radiant energy reflected by the first reflector for approximately reflectively collimating such radiant energy back toward the first reflector; and third reflector means disposed adjacent the center of curvature of the first reflector for receiving radiant energy reflected from the first reflector to the paraxial focus and passed by the aperture in the second reflector and for directing such radiant energy outside of the first reflector.

Other objects of the invention will in part be obvious and will in part appear hereinafter. The invention accordingly comprises the apparatus possessing the construction, combination of elements, and arrangement of parts which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims.

For a fuller understanding of the nature and objects of the present invention, reference should be had to the following detailed description taken in connection with the accompanying drawing wherein there is shown an elevational, sectional view through a schematic embodiment of a telescope of the present invention.

As shown in the drawing, the invention comprises a large concave primary mirror 20 having a substantially spherical curvature for its reflecting surface 22. Mirror 20 preferably is stationary, i.e. fixedly mounted or integral with respect to the earths surface. Because mirror 20 is not subject to gravitational flexure due to changes in orientation as occurs in conventional large primary reflectors, its mounting can be quite simple. Preferably, mirror 20 is supported on a plurality of simple local mounts 24 such as jacks, which are movable to provide static figure adjustment and preferably are constant force supports to reduce the effect of earth tremors.

Mirror 20 thus can have a diameter up to feet. The mirror can be formed of a number of materials, for example of electroformed, specularly reflecting metallic panels, and can be quite light in weight per unit area of reflecting surface because it needs little self-supporting structure.

Mounted preferably immediately above the paraxial focus 26 of primary mirror 20 is aspheric, Gregorian-like secondary reflector 28 having a central aperture 30 therein. The concave reflecting surface 32 of reflector 28 is directed toward the concave surface of mirror 20.

Means such as support 34 are proved for mounting reflector 28 as aforesaid and for pivotal movement about the center of curvature 36 of primary mirror 20. Thus, reflector 28 is movable along an arc concentric with the surface of mirror 20 and through a plurality of positions wherein the optical axis of reflector 28 is colinear with various radii of mirror 20.

The reflecting concave surface of the secondary reflector is figured in known, symmetrically aspheric manner such that light coming from mirror 20 is substantially recollimated back toward the latter, i.e. the light reflected by reflector 28 is in substantially parallel rays or in some instances slightly diverging rays for reasons to be explained hereafter.

Means are further provided for directing, at an angle to the principal axis of mirror 20, paraxial light passing through aperture 30 from mirror 20 in order to observe the light outside of the structure of the telescope. To this end, in the form shown, plane mirror 38 is pivotally mounted at or adjacent center 36 of primary curvature to redirect light preferably along a polar axis, the angle D between the polar axis and principal axis (i.e. the radius of curvature through the center of mirror 20) being the declination.

It will thus be seen that steering of the telescope is accomplished by pivoting the lighter weight optical elements suspended at the center of curvature 36 of mirror 20, although a limited range (about 30) of declinations are available. To describe the operation of the device, one may consider light rays from a distant source (which can be considered collimated light) to be parallel to the principal axis of the primary dish of mirror 20, as by limiting the angular acceptance of the dish by conventional telescope tubing. An exemplary ray 40 (in solid line) incident adjacent the periphery of mirror 20 is focussed approximately toward the paraxial focus 26, and thus is incident on the aspheric surface 32 of the secondary from whence it is redirected, now as an approximately paraxial ray, back toward the primary dish. Those rays closer to the principal axis and not occluded by reflectors 28 or 38 (shown as exemplary ray 42 in broken line) is more tightly focussed toward the paraxial focus and if not passed by aperture 32 is also redirected back toward the primary dish as a more truly paraxial ray. For all paraxial rays reflected toward mirror 20 from reflector 28, mirror 20 then acts as a tertiary reflector and focuses these paraxial rays substantially at focal point 26. The primary, when acting as a tertiary mirror operates at about f/ or slower and thus the return of the light to the paraxial focus is quite sharp, passing readily through aperture 30 to be redirected off of the principal axis of mirror 20, whence it may be subsequently employed. This allows all corrections for aberrations to be made by figuring the surface of the secondary mirror, and the second reflections from the primary mirror acting as a tertiary reflector introduces minimum additional error.

It will be apparent to those skilled in the art that it is not necessary to use mirror 38 but that the light collected by the device, after passage through aperture 30 can be used directly as by positioning an interferometer in place of mirror 38.

If as previously noted, reflector 28 is figured not to recollimate precisely but instead to diverge slightly rays l eflected therefrom, the eventual focus by mirror 20, acting as a tertiary reflector, will lie slightly above, rather than below the secondary reflector. Thus, the focal point will be somewhat more accessible, particularly if one Wishes to substitute instrumentation such as photometer for mirror 38. Regardless of whether reflector 28 collimates or diverges reflected rays, the diameter of reflector 28 is about on the order of three times the diameter of the circle of least confusion of the primary focus.

It should be noted that the system described is useful only as a large light collector, since even slightly offaxis aberrations will be so great as to prevent formation of useful images. The utility of the device therefore lies in photometry and spectroscopy where accurate image formation is usually of considerably less importance than light collection.

It will be appreciated that because mirror 20 is spherically figured there is no primary optical axis in the sense that, for example, a parabolic mirror has such an axis. Thus, the term paraxial as used herein refers to light rays closely adjacent any radius of curvature of the primary mirror which passes through aperture 30, i.e. is colinear with the optical axis of mirror 28.

What is claimed is:

1. A catoptric light collector comprising in combination,

a first concave spherical reflecting surface for receiving and focussing radiant energy, said surface being substantially fixedly mounted with respect to the earths surface; and

an apertured secondary reflector having a concave, re-

flecting surface directed toward said first surface and being aspheric for substantially recollimating radiant energy received from said first reflecting surface back toward the latter, and being positioned adjacent the paraxial focus of said first reflecting surface so that the latter can reflectingly focus the recollimated energy back through the aperture in said secondary reflector, said reflector being mounted for pivotal movement about the center of curvature of said first surface.

2. A light collector as defined in claim 1 including third reflector means for directing outside of the first reflector substantially radiant energy reflected from said spherical surface to said paraxial focus and through said aperture.

3. A light collector as defined in claim 2 wherein said third reflector has a substantially plane reflecting surface mounted adjacent said center of curvature for pivotal movement about the latter.

4. A light collector as defined in claim 1 wherein said secondary reflector and said first reflecting surface are positioned so that said paraxial focus lies between them.

5. A light collector as defined in claim 1 wherein said aspheric secondary reflector is dimensioned in a plane across said reflector about three times the diameter of the circle of least confusion of the primary focus of said first reflecting surface.

References Cited UNITED STATES PATENTS 2,976,5 3 3 3 1961 Salisbury. 3,242,806 3/ 1966 Hine 350-294 FOREIGN PATENTS 370,365 4/ 1939 Italy.

DAVID SCHONBERG, Primary Examiner J. W. LEONARD, Assistant Examiner US. Cl. X.R. 126270 

